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Number Category Status Author Organization Created
0014
Standards Track
Proposal
Xuejie Xiao
Nervos Foundation
2019-01-04

VM Cycle Limits

Introduction

This RFC describes cycle limits used to regulate VM scripts.

CKB VM is a flexible VM that is free to implement many control flow constructs, such as loops or branches. As a result, we will need to enforce certain rules in CKB VM to prevent malicious scripts, such as a script with infinite loops.

We introduce a concept called cycles, each VM instruction or syscall will consume some amount of cycles. At consensus level, a scalar max_block_cycles field is defined so that the sum of cycles consumed by all scripts in a block cannot exceed this value. Otherwise, the block will be rejected. This way we can guarantee all scripts running in CKB VM will halt, or result in error state.

Consensus Change

As mentioned above, a new scalar max_block_cycles field is added to chain spec as a consensus rule, it puts a hard limit on how many cycles a block's scripts can consume. No block can consume cycles larger than max_block_cycles.

Note there's no limit on the cycles for an individual transaction or a script. As long as the whole block consumes cycles less than max_block_cycles, a transaction or a script in that block are free to consume how many cycles they want.

Cycle Measures

Here we will specify the cycles needed by each CKB VM instructions or syscalls. Note right now in the RFC, we define hard rules for each instruction or syscall here, in future this might be moved into consensus rules so we can change them more easily.

Note that right now, the cycles used here aren't carefully tested, what's more, hardcoding cycles here is also a temporary solution. In future we plan to have a different RFC that:

  1. Use numbers that go through more testing and real benchmarks to be more realistic.
  2. Put cycle rules in a cell so we can change them without needing hardforks.

Instruction Cycles

All CKB VM instructions consume 1 cycle except the following ones:

Instruction Cycles
JALR 3
JAL 3
J 3
JR 3
BEQ 3
BNE 3
BLT 3
BGE 3
BLTU 3
BGEU 3
BEQZ 3
BNEZ 3
LD 2
SD 2
LDSP 2
SDSP 2
LW 3
LH 3
LB 3
LWU 3
LHU 3
LBU 3
SW 3
SH 3
SB 3
LWSP 3
SWSP 3
MUL 5
MULW 5
MULH 5
MULHU 5
MULHSU 5
DIV 16
DIVW 16
DIVU 16
DIVUW 16
REM 16
REMW 16
REMU 16
REMUW 16
ECALL 0
EBREAK 0

In a nutshell, the following principles are applied in deciding those numbers:

  • Branches should be more expensive than normal instructions.
  • Memory accesses should be more expensive than normal instructions, but since we are using 64-bit system, accessing 64-bit value should take less time than non 64-bit value.
  • Multiplication and divisions should be much more expensive than normal instructions.

Syscall Cycles

Each syscall in CKB has different rules for consuming cycles:

Load TX

Load TX syscall first consumes 10 initial cycles, it then measures the size of the serialized transaction data: for every single byte in the data, it consumes 10 more cycles.

Note that even though the script only requires part of the serialized TX data, the syscall still charges based on the full serialized data size.

Load TX Hash

Load TX Hash syscall first consumes 10 initial cycles, it then measures the size of the serialized transaction hash(for now, this is 32 bytes): for every single byte in the data, it consumes 10 more cycles.

Note that even though the script only requires part of the serialized TX hash, the syscall still charges based on the full serialized hash size.

Load Cell

Load Cell syscall first consumes 100 initial cycles, it then measures the size of the serialized cell structure data: for every single byte in the serialized data, it consumes 100 more cycles.

Notice the charged cycles here is 10 times the cycles charged in Load Cell By Field syscall, this is because we are discouraging the use of this syscall. One should only use this if they really need the full serialized Cell structure.

Note that even though the script only requires part of the serialized Cell structure data, the syscall still charges based on the full serialized data size.

Load Cell By Field

Load Cell By Field syscall first consumes 10 initial cycles, it then measures the size of the serialized data from the specified field: for every single byte in the serialized data, it consumes 10 more cycles.

Note that even though the script only requires part of the specified serialized field, the syscall still charges based on the full serialized field size.

Load Input By Field

Load Input By Field syscall first consumes 10 initial cycles, it then measures the size of the serialized data from the specified field: for every single byte in the serialized data, it consumes 10 more cycles.

Note that even though the script only requires part of the serialized data, the syscall still charges based on the full serialized data size.

Debug

Debug syscall first consumes 10 initial cycles, it then consumes 10 more cycles for every single byte in the debug parameter string.

Final note

Notice that the two numbers used here (10 and 100) haven't gone through full testing, right now they are picked based on the following principles:

  • We want each syscall to at least consume some cycles even thought some syscall might return no data, hence we add either 10 or 100 initial cycles to each syscall.
  • Syscalls should in general be more expensive than normal instructions to discourage using them unless necessary, hence we are using a scale of 10 or 100 here to make them significantly bigger than most norma instructions.
  • We want to encourage using Load Cell By Field instead of Load Cell, since the former one makes easier implementation and less likely to be attacked, that's why Load Cell syscall use a factor of 100, while Load Cell By Field only use a factor of 10.

In future a different RFC might revise those numbers and even put those rules in a cell for easier changes.

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